1. Field of the Invention
This invention relates to a heat-insulating structure of a swirl chamber in an internal combustion engine and its production method.
2. Description of the Prior Art
Generally, in combustion chambers of a swirl chamber type in an internal combustion engine, mixing of a fuel and air is made twice each in swirl chamber and main combustion chamber and the mixing state is better than in those of a direct injection type. However, the loss of cooling water is greater and the fuel efficiency becomes lower. Therefore, attempts have been made to constitute the swirl chamber in a heat-insulating structure in order to minimize the loss of cooling water. However, if the structure is constituted in such a structure as to thermally insulate the outer surface of each swirl chamber uniformly, durability problem with swirl chambers resulting from the difference of thermal stresses arises.
If the ceramic material constituting each swirl chamber block is silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), or the like, the ceramic material such as silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), or the like, has high heat resistance and can withstand a high temperature and high strength, but has high heat transfer rate and a low heat-insulating property. Since the ceramic material has high Young's modulus and high deformation resistance, a high thermal stress acts on it if any non-uniformity occurs in its temperature distribution.
Further, the temperature distribution of the inner wall surface constituting each swirl chamber is such that the jet port portion for communicating the main combustion chamber with the swirl chamber reaches a high temperature and moreover, the temperature distribution around the jet port portion is such that the jet port portion on the center side of the main combustion chamber reaches particularly a high temperature. Therefore, if each swirl chamber block constituting the inner wall portion of the swirl chamber is made of a ceramic material, the temperature distributions at the jet port portion of the swirl chamber block become considerably nonuniform and the thermal stress therefore acts and exerts adverse influences on the strength of the ceramic material, causing thereby the problem of durability. Accordingly, a problem remains to be solved as to how each swirl chamber itself be constituted in order to improve durability of the swirl chamber block.
Conventionally, the swirl chamber of an engine is disclosed, for example, in Japanese Patent Laid-Open No. 67219/1987. The swirl chamber of the engine disclosed in this reference is equipped with swirl chamber constituent member for forming the swirl chamber and a metallic cylinder member fitted to the outer peripheral portion of the swirl chamber constituent member, and the cylinder member is composed of a martensite type heat-resistant steel having the composition consisting of 0.13 to 0.45 wt % of C, 0.3 to 2.5 wt % of Si, 0.5 to 1.0 wt % of Mn, 10.0 to 13.0 wt % of Cr, 0.3 to 1.3 wt % of Mo and the balance consisting substantially of iron, and converted substantially to a martensite texture. After this cylinder member is shrinkage-fitted to the swirl chamber constituent member, it is then converted to the sorbite structure by tempering.
However, in the swirl chamber of the engine described above, the cylinder member is shrinkage-fitted to the swirl chamber constituent member and then converted to the sorbite structure by tempering. Accordingly, a sufficient residual compressive stress cannot be imparted to the swirl chamber constituent member by the cylinder member and when the swirl chamber constituent member is made of a ceramic material, it cannot withstand the thermal stress resulting from non-uniformity of the temperature distribution. The method of imparting the residual compressive stress to the ceramic material by shrinkage fit of the metallic material to the ceramic material cannot impart effective residual compressive stress to the ceramic material because the imparting direction of the compressive force is unidirectional.
The production method of the swirl chamber of the engine is disclosed, for example, in Japanese Patent Laid-Open No. 83451/1986. The production method of the swirl chamber of the engine disclosed in this reference comprises as follows. When the swirl chamber of the engine is produced by fitting an outer cylinder of an iron type sintered material, that is compression powder molded or is preparatively sintered, to a ceramic inner cylinder, this method comprises the steps of integrating ceramic particles by a copper type bonding material, preparing an insert member molded in the shape substantially equal to the shape of the heat-insulating chamber to be formed at a predetermined position between the inner and outer cylinders, interposing the insert member at the predetermined position between the inner and outer cylinders and subjecting it to the regular sintering step.
In accordance with the production method of the swirl chamber of the engine described above, however, the outer peripheral metal material consists of the sintered member and the sintered metal has the function of only sealing the heat-insulating layer but cannot control the compressive force to the ceramic inner cylinder and cannot either control the coefficient of thermal expansion and thermal transfer rate of the outer cylinder. Therefore, this prior art reference does not have the technical concept of improving durability of the ceramic inner cylinder.
Furthermore, when the block that constitutes the swirl chamber is directly casted into a large-scale component such as the cylinder head, deviation of dimension becomes excessive at the time of casting and the resulting product cannot be used as an approved product. The deviation of the casting dimension is about .+-.1.5 mm for the size of about 500 mm, for example, but accuracy of the position dimension of the swirl chamber must be about .+-.0.2 mm.